1. Field of the Invention
This invention relates to a heat exchanger apparatus comprising heat exchangers in which a porous material through which a fluid, such as an exhaust gas and vapor can pass is packed in passages, and a ceramic engine provided with a supercharger driven by the thermal energy recovered from an exhaust gas by the same heat exchanger apparatus.
2. Description of the Prior Art
A conventional turbocharger-carrying heat insulating engine is provided in a first stage of an exhaust system with a turbine-and-compressor-carrying turbocharger, and on the downstream side of the turbocharger with an energy recovery unit comprising a generator-carrying turbine. In the heat insulating engine, a combustion chamber is formed with a heat insulating structure, and the thermal energy of an exhaust gas discharged from the combustion chamber is recovered as electric power by the turbocharger and energy recovery unit, this thermal energy being otherwise recovered by being supercharged to the engine by the operations of the turbocharger and compressor.
An example of an energy recovery system formed so as not to lower the recovery efficiency of the exhaust gas energy with respect to such a heat insulating engine is disclosed in Japanese Patent Laid-Open No. 179972/1993. This energy recovery system has an energy recovery unit provided with a first turbine installed in an exhaust passage and a generator operated by the first turbine, a turbocharger provided with a second turbine connected to an outlet-side passage of the first turbine and a supercharging compressor operated by the second turbine, and a waste gate provided in the outlet-side passage of the first turbine. Since an energy recovering operation is carried out by the energy recovery unit when the temperature of the exhaust gas is high, the recovering of the energy is done effectively.
In a cogeneration system, the power is taken out as electric energy by a generator, and water is heated with electric power based on the thermal energy of an exhaust gas and a heat exchanger provided in an exhaust passage to produce hot water, which is utilized as a hot water supply source. Such a cogeneration system is subjected to a rated operation in which the load fluctuation is small, so that it is expected to be utilized as a power supply system in an urban area and a mountainous area.
Such a cogeneration system is disclosed in, for example, Japanese Patent Laid-Open No. 33707/1994. The cogeneration engine is adapted to generate steam by the exhaust gas energy, and improve the thermal efficiency by recovering the steam energy as electric energy. A turbocharger is driven by the exhaust gas energy from a heat insulating gas engine, and a generator-carrying energy recovery unit is driven by the exhaust gas energy from the turbocharger. The thermal energy of an exhaust gas from the energy recovery unit is converted into steam by a first heat exchanger, and recovered as electric energy by driving a steam turbine by the mentioned steam. Furthermore, hot water is generated by the high-temperature steam from the steam turbine by an operation of a second heat exchanger, and utilized as a hot water supply source.
A heat exchanger has to change its structure depending upon gas-phase and liquid-phase heat exchanging substances. When the structure is complicated, or, when consideration is not given to the structure with respect to the strength thereof, a crack or breakage occurs in the heat exchanger. In order to improve the thermal efficiency in a heat exchanger by increasing the temperature of the suction gas in, for example, a gas turbine, exchanging of heat between a gas and a gas has to be effected. In general, a heat exchanger is formed with a structure having a rotating cylindrical honeycomb unit, and capable of heating an intake gas with an exhaust gas, whereby the intake gas can receive heat. This type of heat exchanger is effective to exchange heat between an exhaust gas and an intake gas. However, the construction becomes complicated, and cracks occur in a wall intake surface to cause an exhaust gas to be mixed in the intake gas.
In the above-described heat insulating engine, a combustion chamber is formed with a heat insulating structure, and a turbocharger and an energy recovery turbine are disposed in series. A high exhaust back pressure occurs in an exhaust stroke of a piston, i.e., a pressure against the discharging of an exhaust gas occurs, so that applying a discharge pressure to the exhaust gas is required in which negative work is carried out under compulsion. For example, in a heat insulating engine, pressure ratios in the turbocharger and energy recovery turbine are 2 respectively, and, assuming that the pressure at an outlet portion of the energy recovery turbine is atmospheric pressure, the back pressure at an outlet of the combustion chamber of the engine reaches 2.times.2=4 (kg/cm.sup.2), which constitutes negative work in an exhaust stroke of the piston and becomes a loss.
Since an exhaust gas discharged from a combustion chamber has not only a pressure but also thermal energy, it is necessary to recover the thermal energy from the exhaust gas effectively.
When a turbine-side efficiency and a compressor-side efficiency in a turbocharger are, for example, around 80% respectively, a recovery efficiency attained by the turbine and compressor is 0.8.times.0.8=0.64(=64%). When an energy recovery-side turbine efficiency and a conversion efficiency are 0.8 and 0.85 respectively, a total recovery efficiency becomes 0.68.
When the efficiency of the turbocharger is taken into consideration, the thermal energy therein is high since the temperature of the exhaust gas is high. When the work conversion rate of exhaust thermal energy is set twofold, the energy recovered at the suction side is 2.times.2.times.0.64=2.56 (kg/cm.sup.2). However, since 2 kg/cm.sup.2 is consumed as negative work due to a back pressure, the gain of the energy recovered by the turbocharger is 2.56-2=0.56 (kg/cm.sup.2).
When the efficiency of the energy recovery turbine which includes a rate of converting the thermal energy into power is twofold, the recovered energy is 2.times.2.times.0.68=2.72 (kg/cm.sup.2). However, since 2 kg/cm.sup.2 is consumed as negative work due to a back pressure, the gain of the energy recovered by the energy recovery turbine is 2.72-2=0.72 (kg/cm.sup.2).
Accordingly, the gain of the total energy recovered by the turbocharger and energy recovery turbine is 0.56+0.72=1.28 (kg/cm.sup.2). Therefore, even when consideration is given to the suction boosting of the turbocharger and energy recovery efficiency, a loss due to a back pressure reaches a high level.
Referring to a PV diagram of FIG. 6, the following may be understood. In a conventional engine provided with a turbocharger and an energy recovery turbine, an exhaust valve is opened at the beginning ES of an exhaust stroke, and an exhaust gas blows down BD and is discharged to an exhaust passage until a terminal end ET of the exhaust stroke as shown in a hatched portions EIS of the PV diagram of FIG. 6. The pressure and thermal energy (5.2 kg/cm.sup.2) of the exhaust gas are recovered as turbine work by the turbocharger and energy recovery turbine. However, the exhaust gas is forced out from the beginning ES of an exhaust stroke to a terminal end ET thereof, and the air is sucked into a combustion chamber from the beginning IS of an intake stroke to a terminal end IT thereof, so that the hatched portion EIS of the PV diagram constitutes negative work.